Like any other type of phytoplankton,
coccolithophores are one-celled marine
plants that live in large numbers throughout the upper layers of the ocean.
Unlike any other plant in the ocean, coccolithophores surround themselves with a
microscopic plating made of limestone (calcite). These scales, known as
coccoliths, are shaped like hubcaps and are only three one-thousandths of a
millimeter in diameter.

What coccoliths lack in size they make up in volume.
At any one time a single coccolithophore is attached to or surrounded by at
least 30 scales. Additional coccoliths are dumped into the water when the
coccolithophores multiply asexually, die or simply make too many scales. In
areas with trillions of coccolithophores, the waters will turn an opaque
turquoise from the dense cloud of coccoliths. Scientists estimate that the
organisms dump more than 1.5 million tons (1.4 billion kilograms) of calcite a
year, making them the leading calcite producers in the ocean.

The picture at top shows the large numbers of detached coccoliths gathered by filtering the ocean with a fine mesh during a bloom.
(Micrograph courtesy Jeremy Young)

Most phytoplankton need both sunlight and
nutrients from deep in the ocean. The ideal place for them is on the surface of
the ocean in an area where plenty of cooler, nutrient-carrying water is
upwelling from below. In contrast, the coccolithophores prefer to live on the
surface in still, nutrient-poor water in mild temperatures.

Coccolithophores
do not compete well with other phytoplankton. Yet unlike their cousins,
coccolithophores do not need a constant influx of fresh food to live. They
often thrive in areas where their competitors are starving. Typically, once
they are in a region, they dominate and become more than 90 percent of the
phytoplankton in the area.

Coccolithophores live mostly in subpolar regions.
Some other places where blooms occur regularly are the northern coast of
Australia and the waters surrounding Iceland. In the past two years, large
blooms of coccolithophores have covered areas of the Bering Sea. This surprises
many scientists since the Bering Sea is normally a nutrient-rich body of water.

The photograph at upper left shows the color of water containing a coccolithophore bloom.
The scientists are using a very fine net to collect Emiliania huxleyi, a species of coccolithophore.
(Photograph courtesy Patrick Holligan)

Coccolithophores are
not normally harmful to other marine life in the ocean.
The nutrient-poor conditions that allow the coccolithophores to exist will
often kill off much of the larger phytoplankton. Many of the smaller fish and
zooplankton that eat normal phytoplankton also feast on the coccolithophores.
In nutrient-poor areas where other phytoplankton are scarce, the
coccolithophores are a welcome source of nutrition.

In the long term, the
plants seem to be good for the environment. Coccolithophores make their
coccoliths out of one part carbon, one part calcium and three parts oxygen
(CaCO3). So each time a molecule of coccolith is made, one less carbon atom is
allowed to roam freely in the world to form greenhouse gases and contribute to
global warming. Three hundred twenty pounds of carbon go into every ton of
coccoliths produced. All of this material sinks harmlessly to the bottom of the
ocean to form sediment.

The coccolithophores' short-term effect on the
environment is somewhat more complex. This effect again has to do with the
formation of their coccoliths and the chemical reaction involved in the process.
The chemical reaction that makes the coccolith also generates a carbon dioxide
molecule, a potent greenhouse gas, from the oxygen and carbon already in the
ocean. While much of the gas is sucked back in by the coccoliths (all plants
take in carbon dioxide for food) some of it escapes into the atmosphere and
immediately becomes part of the greenhouse gas problem. Scientists are
concerned in the short term that greenhouse gases will cause the upper layers of
the ocean to become more temperate and stagnant. This would increase the number
of coccoliths in the world, which would produce more greenhouse gas.

The
coccolithophores also affect the global climate in the short term by increasing
the oceans' albedo. Albedo is the fraction of sunlight an object
reflects--higher albedo values indicate more reflected light. Coccolithophore
blooms reflect nearly all the visible light that hits them. Since most of this
light is being reflected, less of it is being absorbed by the ocean and stored
as heat.

Above is an image taken from space, showing
a coccolithophore bloom south of Iceland. Ocean currents and eddies can be seen in the swirls
of aquamarine water. (Image courtesy Norman Kuring, SeaWiFS project)